Your need to make two rectangles. Say you have a 10 m chimney with a fire 1 m high in the bottom, and a width of 0.1 m. You make one area from z = 0 m to z = 1 m and a second area from z = 1 m to z = 10 m. The two must share a line at the 1 m height. Mesh the two areas. Now FLUENT will recognize two separate fluid regions. You can specify the heat in the lower region.

Buoyancy flows are not always easy to converge. First, look at your mesh. Mesh density can have a big influence. If continuity is oscillating, you may need more cells. Second, look at your under-relaxations. Use 0.98 for energy and 0.8 for the body force and density. Third, reconsider the definition of converged. The default parameters in FLUENT are pretty good for most cases, but are as arbitrary as any other criteria you might choose to apply.